Page 403 - Air and Gas Drilling Manual

Chapter 9: Aerated Fluid Drilling 9-7
9.2.3 Advantages and Disadvantages
Given below are the advantages and disadvantages of the drill pipe injection
technique.
The advantages to the drill pipe injection technique are as follows:
• The technique does not require any additional downhole equipment,
therefore, drill pipe injection is less costly than annulus injection.
• Nearly the entire annulus is filled with aerated fluid, thus, lower
bottomhole pressures can be relative to annulus injection.
• Since the gas is injected into the annulus at or near the bottom of the
annulus, less gas volumetric flow rate is needed to achieve a given
bottomhole pressure than via annulus injection.
The disadvantages to the drill pipe injection technique are as follows:
• Aeration of the incompressible drilling fluid cannot be continued when
circulation is discontinued for connections and tripping. Therefore, it
is difficult to maintain underbalanced drilling operations.
• Since the injected gas is trapped under pressure inside the drill string by
the various string floats, time must be allowed for the pressure bleed-
down when making connections and trips. Here again the bleed-down
makes it difficulty to maintain a constant bottomhole
pressure.
• The flow down the inside of the drill string is two phase flow and,
therefore, high pipe friction losses are present. The high friction losses
results in high pump and compressor pressures during injection.
• The gas phase in the aerated flow attenuates the pulses of conventional
(measure-while-drilling) MWD systems. Therefore, conventional mud
pulse telemetry MWD cannot be used.
Given below are the advantages and disadvantages of the annulus injection
technique.
The advantages to the annulus injection technique are as follows:
• Aeration of the incompressible drilling fluid in the annulus above the gas
annulus entrance position can continue during connections and trips.
• Flow down the inside of the drill string is single phase, therefore,
conventional mud pulse MWD can be used in aerated directional drilling
operations.
• The compressor pressure to maintain injection flow will usually be low
when compared to the pressure required for direct circulation (to the drill
bit) aerated drilling operations.
The disadvantages to the annulus injection technique are as follows:
• Because the parasite tubing string or the temporary casing are placed at a
particular fixed location in the well, the aeration technique is more
inflexible than the drill pipe injection technique.
• Initiating (kickoff) gas flow to the annulus requires very high compressor
pressures.
• Since the gas is injected into the annulus at fixed depths that are well
above the bottom of the annulus, higher gas volumetric flow rates are
required to maintain constant bottomhole pressures than in the drill pipe
injection technique.